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Tyler Janicke
Theoretical work by Hideki Yukawa in 1935 had predicted the existence of mesons as the carrier particles of the strong nuclear force. From the range of the strong nuclear force (inferred from the radius of the atomic nucleus), Yukawa predicted the existence of a particle having a mass of about 100 MeV/c2. Initially after its discovery in 1936, the muon (initially called the "mu meson") was thought to be this particle, since it has a mass of 106 MeV/c2. However, later experiments showed that the muon did not participate in the strong nuclear interaction. In modern terminology, this makes the muon a lepton, and not a meson. However, some communities of astrophysicists continue to call the muon a "mu-meson".[according to whom?] The pions, which turned out to be examples of Yukawa's proposed mesons, were discovered later: the charged pions in 1947, and the neutral pion in 1950.
In 1947, the first true mesons, the charged pions, were found by the collaboration led by Cecil Powell at the University of Bristol, in England. The discovery article had four authors: César Lattes, Giuseppe Occhialini, Hugh Muirhead and Powell.[3] Since the advent of particle accelerators had not yet come, high-energy subatomic particles were only obtainable from atmospheric cosmic rays. Photographic emulsions based on the gelatin-silver process were placed for long periods of time in sites located at high-altitude mountains, first at Pic du Midi de Bigorre in the Pyrenees, and later at Chacaltaya in the Andes Mountains, where the plates were struck by cosmic rays.After development, the photographic plates were inspected under a microscope by a team of about a dozen women.[4] Marietta Kurz was the first person to detect the unusual "double meson" tracks, characteristic for a pion decaying into a muon, but they were too close to the edge of the photographic emulsion and deemed incomplete. A few days later, Irene Roberts observed the tracks left by pion decay that appeared in the discovery paper. Both women are credited in the figure captions in the article.
In 1948, Lattes, Eugene Gardner, and their team first artificially produced pions at the University of California's cyclotron in Berkeley, California, by bombarding carbon atoms with high-speed alpha particles. Further advanced theoretical work was carried out by Riazuddin, who in 1959 used the dispersion relation for Compton scattering of virtual photons on pions to analyze their charge radius.[5]
The use of pions in medical radiation therapy, such as for cancer, was explored at a number of research institutions, including the Los Alamos National Laboratory's Meson Physics Facility, which treated 228 patients between 1974 and 1981 in New Mexico,[8] and the TRIUMF laboratory in Vancouver, British Columbia.
In the standard understanding of the strong force interaction as defined by quantum chromodynamics, pions are loosely portrayed as Goldstone bosons of spontaneously broken chiral symmetry. That explains why the masses of the three kinds of pions are considerably less than that of the other mesons, such as the scalar or vector mesons. If their current quarks were massless particles, it could make the chiral symmetry exact and thus the Goldstone theorem would dictate that all pions have a zero mass.
With the addition of the strange quark, the pions participate in a larger, SU(3), flavour symmetry, in the adjoint representation, 8, of SU(3). The other members of this octet are the four kaons and the eta meson.
The second most common decay mode of a pion, with a branching fraction of 0.000123, is also a leptonic decay into an electron and the corresponding electron antineutrino. This "electronic mode" was discovered at CERN in 1958:[11]
Its mechanism is as follows: The negative pion has spin zero; therefore the lepton and the antineutrino must be emitted with opposite spins (and opposite linear momenta) to preserve net zero spin (and conserve linear momentum). However, because the weak interaction is sensitive only to the left chirality component of fields, the antineutrino has always left chirality, which means it is right-handed, since for massless anti-particles the helicity is opposite to the chirality. This implies that the lepton must be emitted with spin in the direction of its linear momentum (i.e., also right-handed). If, however, leptons were massless, they would only interact with the pion in the left-handed form (because for massless particles helicity is the same as chirality) and this decay mode would be prohibited. Therefore, suppression of the electron decay channel comes from the fact that the electron's mass is much smaller than the muon's. The electron is relatively massless compared with the muon, and thus the electronic mode is greatly suppressed relative to the muonic one, virtually prohibited.[12]
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High School
Earned four letters under head coach Michelle McGuire at the Windward School ... Played outside hitter for the Wildcats ... Was an SCVA Second Team All-American ... Was a four-time All-League selection ... Was twice named Alpha League MVP ... Twice was selected to the All-CIF Team ... Has the school and league record for single-match kills with 57 against Paraclete in his senior year ... Won league championships in 2011, 2012, and 2014 ... Made it to CIF Playoffs all four years, including semifinal appearances in 2012 and 2014 and a Southern California championship in 2012 ... Also competed in football and soccer ... Was a three-time All-League selection in soccer ... Won a 2011 CIF Championship in football ... Made the honor roll all four years.
We present a new, comprehensive global analysis of parton-to-pion fragmentation functions at next-to-leading-order accuracy in QCD. The obtained results are based on the latest experimental information on single-inclusive pion production in electron-positron annihilation, lepton-nucleon deep-inelastic scattering, and proton-proton collisions. An excellent description of all data sets is achieved, and the remaining uncertainties in parton-to-pion fragmentation functions are estimated based on the Hessian method. Extensive comparisons to the results from our previous global analysis are performed.
Comparison of our NLO results for charged pion multiplicities in SIDIS off proton (left panels) and deuteron (right panels) targets with data from the HERMES Collaboration [30]. The inner and outer shaded bands correspond to uncertainty estimates at 68% and 90% C.L., respectively. Also shown are the results obtained with the DSS FFs (dashed lines).
A comprehensive review is presented dealing with pion excitations and condensation in nucleon matter. A discussion of the behavior of bosons in scalar electric and nuclear fields is given along with a consideration of the possible existence of superdense and supercharged nuclei. The applications to nuclei and neutron stars are given.
A meson occurring either in a neutral form with a mass 264 times that of an electron and a mean lifetime of 8.4 X 10-17 seconds or in a positively or negatively charged form with a mass 273 times that of an electron and a mean lifetime of 2.6 X 10-8 seconds. The pion was once believed to be the particle that mediates the strong force, which holds nucleons together in the nucleus; it is now believed that the gluon is the mediator particle. Pions do interact with nucleons, however, and are able to transform neutrons into protons and vice versa. Also called pi-meson See Table at subatomic particle.
A new Monte Carlo QCD analysis of pion parton distributions has been performed by the Jefferson Lab Angular Momentum collaboration (JAM), including for the first time the transverse momentum (pT) dependent pion-nucleus Drell-Yan cross sections together with pT -integrated Drell-Yan and leading neutron electroproduction data from HERA. The study assesses the sensitivity of the Monte Carlo fits to kinematic cuts, factorization scale and parametrization choice, and it discusses the impact of the various data sets on the pion's quark and gluon distributions. The work provides the necessary step toward the simultaneous analysis of collinear and transverse momentum dependent pion distributions and the determination of the pion's three-dimensional structure.
There are six types of quark (called flavours) but only two flavours go together to make a pion. These flavours are called up and down. Quarks have charge, so two quarks of the same flavour (both up or both down) make a neutral pion. But when the two quarks have different flavours (up and down), the pion will have a charge. This charge is positive when an up quark pairs with a down antiquark. The charge is negative when a down quark pairs with an up antiquark.
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